The economics of EPDM resin processes depend greatly on the effectiveness of the diene monomer recovery system. The diene monomer constitutes a small part of the polymer composition, typically about 2% to 8%. However, the polymer concentration of unreacted diene is substantial. At approximately five times the cost of the propylene monomer, a few percent of unrecovered diene monomer can have a devastating effect on the economic viability of the process. Conversely, the high value of the diene monomer justifies the investment and operating costs of equipment dedicated to its recovery and reuse in the reactor.
In general, current production facilities for producing EPDM resin are based on slurry or solution processes. In these processes, the polymer accounts for only a small fraction of the total weight of the reactor effluent. For example, in the solution process, the polymer fraction is typically less than 10% while the composition of the remaining 90% is a mixture of solvent and liquid monomers.
Typically, in a solution process, unreacted monomers and solvent are removed in three steps. In the first step, the solution is pumped and flashed into a vessel where the lighter hydrocarbons are vaporized. A substantial amount of unreacted diene monomer still remains in the polymer because of its higher molecular weight and its greater affinity to the polymer. Ethylene and most of the propylene are extracted during this first pass.
In a second step, a pump pressurizes the polymer enriched solution through an atomizer to create fine droplets. The atomization produces two major benefits. First, it promotes faster evaporation rates of the diluent by creating larger surfaces for mass transfer with the breakup of the fluid in small droplets. Second, as the evaporation proceeds, the solid fraction in the droplets increases, yielding ultimately fine particles of highly concentrated polymer. The creation of these fine particles is critical for the last treatment step which consists of collecting these particles in a water bath maintained at high temperature by bubbling steam through the water. The purpose of the water is to increase the temperature of the particles to accelerate the desorption rate of the diene monomer residues. This is necessary because dienes used in the fabrication of EPDM resins have typically high boiling points and have a much lower mobility than the other components in the polymer matrix. In the literature, the process just described is commonly referred to as steam stripping. An example of this process is disclosed in British patent 1,104,740. Other examples abound in relation to the process of steam stripping vinyl (VC) from polyvinyl chloride (PVC) such as disclosed in British patent 1,577,381.
Once in the water bath, the fine particles agglomerate into larger ones of irregular shape due to the stickiness of the product at this temperature. The agglomerates or crumbs collect at the bottom of the vessel under the effect of gravity. They are subsequently retrieved, screened and pressed to remove the excess water. The product is then baled and packaged.
The recovery of the solvent, monomers and dienes is made by multiple separation columns which are simple in concept but substantial in size due to the sheer volume of liquids and gas to be treated.
More recently, EPDM resins are also being produced by a gas phase process such as disclosed in U.S. Pat. No. 4,994,534. The gas phase production of EPDM resins results in a product morphology similar to other alpha-olefins produced in gas phase reaction. The polymer is obtained in particle form with an average particle size ranging from about 0.015 to 0.04 inch and a particle size distribution typical for this process. The comparatively larger size of the particles renders inefficient the steam stripping process described earlier.
Gas phase reactor products are polymerized with high activity catalysts. The low catalyst residues remaining do not warrant an extraction step and are readily deactivated by means of additives. They also require only a mild treatment to remove unreacted monomers and other inert hydrocarbons dissolved in the polymer.
Thus in the gas phase process, the removal of the unreacted monomers and solvent dissolved in the polymer is based on contacting the polymer with an inert gas such as nitrogen for a period of time. Descriptions of these processes abound in the literature.
The problem becomes even more significant when ethylidene norbornene (ENB) is used as a monomer. ENB is intentionally added to the reaction system to produce terpolymers of ethylene and propylene. Once incorporated on the polymer chain backbone, ENB supplies a pendant double bond available for cross-linking the network of polymer chains under sulfur cure conditions.
The necessity of removing ENB is many fold. ENB is a flammable hydrocarbon which should be removed from the product for safety reasons. In addition, ENB has a very distinctive and unpleasant odor with a very low human detection threshold. Finally, the removal of ENB and its reuse in the reactor is critical to the economics of the manufacturing process of EPDM resins.
ENB has a molecular weight of 120 and a normal boiling point of 148.degree. C. Compared to a lighter hydrocarbon often encountered in the production of alpha olefin polymers such as 1-hexene, ENB requires 6 to 10 times longer to desorb from an EPDM resin under equivalent processing conditions.
In addition to having slower desorption rates than the usual olefins and solvent encountered in gas phase operation, the driving force to initiate desorption is comparatively very weak as indicated by the high solubility of ENB vapors in contact with an EPDM polymer.
The significance of the intrinsic diffusivity and solubility properties of the ENB and polymer system has major implications on the design of processes aimed at removing ENB residues from the polymer particles. Under the best of circumstances, the purging facilities will be large compared to other processes designed to remove lower boiling point monomers resulting in a substantial increase in investment costs.
The high solubility of ENB has an even larger impact. To desorb, the partial pressure of ENB in the gas phase must be substantially lower than its equilibrium pressure. The equilibrium pressure in the context of desorption is defined as the pressure needed to maintain a given dissolved concentration of ENB in the polymer. For this reason, a large flow of purge gas is needed to effect the desorption of ENB. The gas exits the purger with a very low concentration of ENB in the gas phase. ENB high dilution and the large amount of purge gas utilized per pound of processed ENB has necessitated a separation system that translates in a costly investment and a high operation cost.
It is therefore an object of the present invention to provide a process designed to remove diene monomers, particularly ENB, from EPDM resins in an economical manner. Other objects and advantages of the present invention will become apparent as the invention and the problem that it addresses are described in more detail.